The yellow-pigmented bacterium isolated from a ditch was a Gram negative rod with a G+C content of 63 mol%, and was classified in the genus Sphingomonas. Electron microscopy revealed that the bacterial cell surface was covered with many large plaits. When grown in a medium containing a polysaccharide as an essential nutrient, a pit of 0.02 similar to 0.1 mu m in diameter was formed on the cell surface, and a thin section showed the rearrangement of the plaits and the presence of a region where the cell membrane sinks into the cytosol. The dependence of the pit formation on the presence of macromolecule may predict the existence of a direct uptake mechanism for macromolecules through a mouth-like pit, possibly in endocytosis fashion. The confirmation of the pit structure is the first such finding in the history of microbiology and may provide a new insight into the cell morphology and biochemistry of macromolecule transport in microbial cell system. (C) 1996 Academic Press, Inc.